A lithographic apparatus is a machine constructed to apply a desired pattern onto a substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). A lithographic apparatus may for example project a pattern from a patterning device (e.g. a mask) onto a layer of radiation-sensitive material (resist) provided on a substrate. This may be achieved using a projection system.
The wavelength of radiation used by a lithographic apparatus to project a pattern onto a substrate determines the minimum size of features which can be formed on that substrate. A lithographic apparatus which uses EUV radiation, being electromagnetic radiation having a wavelength within the range 4-20 nm, may be used to form smaller features on a substrate than a conventional lithographic apparatus (which may for example use electromagnetic radiation with a wavelength of 193 nm).
It is desirable to increase the accuracy with which a lithographic apparatus applies a desired pattern to a substrate and to reduce optical aberrations that can cause distortion of the pattern.
It is an object of the present invention to provide a projection system for a lithographic apparatus that at least partially addresses one or more of the problems of the prior art, whether identified herein or elsewhere.